1. Field of the Invention
The present invention relates to a wireless communication system, a wireless base station, a transmission rate determining method, and a program for wireless communication system supporting a plurality of transmission rates in wireless communication.
2. Description of the Related Art
A wireless communication system such as a wireless Local Area Network (LAN) has carried out multi-rate communication in which a plurality of transmission rates is supported.
For example, IEEE802.11b standard prescribes four transmission rates including 11 Megabits per second (Mbps), 5.5 Mbps, 2 Mbps, and 1 Mbps for the wireless LAN. IEEE802.11a standard defines eight transmission rates, namely, 54, 48, 36, 24, 18, 12, 9, and 6 Mbps in which 54, 48, 36, 28, and 9 Mbps are optional.
The multi-rate communication has a basic aspect that if the transmission rate becomes lower, the communicable distance can be elongated. Therefore, as shown in FIG. 1, the wireless base station communicates with a nearby wireless terminal at a higher transmission rate. For farther wireless terminals, the base station uses a lower transmission rate, thereby conducting efficient wireless communication in a wide area.
In general, when the transmission rate dynamically varies in the multi-rate communication, it is difficult to correctly calculate a band to be employed or occupied for each communication. Therefore, to secure the quality of sound and voice in the wireless communication especially when a wireless audio terminal is utilized, there has been used a method to fix a transmission rate, as shown in FIG. 1, that is assigned according to electric wave intensity.
The transmission rates to be allocated by the base station are determined beforehand using values obtained through experiences.
In this connection, for example, document 1 (Japanese Patent Application Laid-Open No. 11-331123) describes a technique for a conventional Code Division Multiple Access (CDMA) communication system in which, if a base station recognizes that the traffic for communication through the base station has a margin, the base station establishes a plurality of communication links with mobile stations.
However, in the conventional wireless communication system employing the multi-rate communication, the connection is established using a transmission rate determined beforehand according to the wave intensity. Therefore, even if the traffic is, for example, small and hence the connection capacity of the wireless base station has a margin, the connection is disadvantageously established using the predetermined transmission rate. That is, even in a situation in which the connection can be made using a higher transmission rate if the connection capacity of the base station is fully used, the connection is established by use of the transmission rate determined beforehand according to the wave intensity.
According to the technique described in document 1, the communication speed is increased using a single transmission rate for wireless communication. However, consideration has not been given to an operation to establish connection using a higher transmission rate by more effectively conducting the connection control for each transmission rate.